1. Field of Invention
The present invention relates generally to solar power systems for space satellite applications. More specifically, this invention relates to solar concentrators for space power, particularly lightweight inflatable solar concentrators for photovoltaic and/or solar thermal energy conversion systems.
2. Description of Prior Art
Prior art solar electric power supplies for space satellites use large flat plate arrays of solar cells made from silicon, gallium arsenide, or another semiconductor material. These prior art flat plate solar arrays are generally very expensive due to the large area of semiconductor material required. These prior art solar arrays are generally heavy due to the combined mass of solar cell, cover glass, and backup structure. These prior art arrays are generally bulky and fragile, complicating their packaging for launch into space. These prior art arrays are also complex to deploy on orbit, requiring mechanical means to unfold the arrays and properly position them for operation.
To minimize or overcome these problems, new approaches to solar electric power supplies for space satellites have recently been developed by Fraas and O'Neill, U.S. Pat. No. 5,344,497 and U.S. Pat. No. 5,505,789. The new approaches use Fresnel lenses to collect and focus sunlight onto very high efficiency tandem-junction solar cells. By using a relatively inexpensive Fresnel lens to collect the sunlight and to focus it onto much smaller solar cells, the cost and weight of the cells are dramatically reduced. By using very high efficiency cells, the required array area is minimized, reducing overall system weight and launch volume. The advantages of the new Fresnel lens concentrating solar electric power supply are now being widely recognized, as witnessed by the selection of this approach for the NASA Jet Propulsion Laboratory's New Millennium Deep Space One satellite scheduled for launch in July 1998. The Fresnel lens concentrating solar array on Deep Space One will provide not only the power for the satellite, but also the power for the electric propulsion system which will propel the probe to encounters with an asteroid, the planet Mars, and a comet.
Despite the many advantages of the Fresnel lens concentrating solar array previously invented by O'Neill and Fraas, this array still has shortcomings in need of improvement. Specifically, the Fresnel lens is presently made from a space qualified, optically clear silicone rubber material (e.g., Dow Corning DC 93-500). This thin rubber lens (e.g., 250 microns thick) must be laminated to a thin (e.g., 80 microns thick) ceria-doped glass superstrate to maintain the required arch shape of the lens assembly. The glass is a structural component, not required for the optical functioning of the lens. Unfortunately, the glass increases the weight, cost, launch volume, and fragility of the lens. Until now, if the glass were not used, the lens would not maintain its shape, even in the zero-gravity environment of space. The presently used glass/silicone Fresnel lens also requires a supporting structure to properly position the lens above the solar cells. This lens support structure adds further weight, cost, and complexity to the solar power system.
To overcome these problems with the prior art Fresnel lens solar concentrator, I have invented an inflatable Fresnel lens which provides all of the benefits of solar concentration, while overcoming the cost, weight, launch volume, and fragility problems of the prior art approach.
Prior art solar thermal power supplies for space satellites use parabolic mirrors to focus sunlight onto solar thermal energy receivers. The high-temperature heat produced by the focussed sunlight can be used for solar thermal propulsion, or for solar dynamic energy conversion, using heat engines based on a Brayton cycle, Stirling cycle, Rankine cycle, or other thermodynamic cycle. These prior art solar thermal power supplies require extremely accurate parabolic mirror surfaces to achieve acceptably high concentration of the solar energy. Prior art solar concentrators for solar thermal power supplies are expensive and heavy, due to the high accuracy requirements of the mirrors.
To minimize or overcome these problems with prior art solar concentrators for solar thermal power in space, an Inflatable Antenna Experiment (IAE) was developed by NASA et al. and launched from the U.S. Space Shuttle in May 1996. This lightweight inflatable parabolic mirror demonstrated mirror surface accuracy that is adequate for radio frequency communication applications, but not adequate for solar concentrator applications. To overcome these problems with the prior art concentrators for solar thermal applications in space, I have invented a second embodiment of the inflatable Fresnel lens which provides all of the benefits of high concentration, while overcoming the cost, weight, and severe accuracy requirements of the prior art parabolic mirror approaches.
While other inventors have proposed approaches to inflatable concentrators for space power in the past, all of these approaches have had substantial shortcomings. For example, Clemens, U.S. Pat. No. 5,660,644, proposed an inflatable reflective concentrator for space power. However, all reflective concentrators require a much higher degree of surface accuracy than an optimized refractive concentrator, as discussed by O'Neill in Chapter 10 of the textbook, Solar Cells and their Applications, published by John Wiley in 1995. An optimized refractive concentrator corresponds to the Fresnel lens configuration taught by O'Neill in U.S. Pat. No. 4,069,812. Since reflective concentrators need more than 100 times better shape accuracy than an optimized refractive concentrator, the reflective devices will be more expensive, heavier, and lower performing, than optimized refractive concentrators.
Sleeper, U.S. Pat. No. 3,125,091, proposed an inflatable refractive concentrator for ground or space power applications. The Sleeper configuration relied on a full cylinder inflated structure for line-focus applications, and a full spherical inflated structure for point-focus applications. The full cylinder and full sphere approaches of Sleeper required the energy receiver/converter to be located either entirely inside or entirely outside of the inflatable concentrator, complicating the positioning and support of these key receiver/converter components. Furthermore, removing waste heat from the interior of Sleeper's concentrators was difficult, requiring bulky coolant fluid conduits to penetrate the ends or sides of the inflated concentrator.
In contrast to the Sleeper approach, my new invention integrates the receiver/converter with the back surface of the inflatable concentrator. The back surface of my invention is thereby able to provide simple positioning and support for the receiver/converter. Furthermore, unlike the Sleeper approach, the back surface of my invention can also serve as the waste heat radiator for the concentrator. Furthermore, unlike the Sleeper approach, the back surface of my invention can be flatter and stronger than the flexible cylindrical or spherical front and side surfaces of the concentrator. This flatter, stronger back surface can also provide a convenient, low-volume platform for stowing the flexible lens and sides of the deflated concentrator for launch into space. Still furthermore, my invention is compatible with the optimized refractive concentrator approach taught by O'Neill, U.S. Pat. No. 4,069,812. Since this optimized lens maximizes both optical efficiency and shape error tolerance, it is ideally suited for an inflatable concentrator application.
The dramatic extent of the improvement provided by my invention is described by O'Neill and Piszczor in "Inflatable Lenses for Space Photovoltaic Concentrator Arrays," in the Proceedings of the 26th IEEE Photovoltaic Specialists Conference, October 1997. While the present state of the art in space photovoltaic arrays is represented by a power-to-mass ratio, also called specific power, of about 50 Watts per kilogram, the new inflatable concentrator should extend this critical system performance index to 400 Watts per kilogram, an eight-fold gain. This extraordinary prediction is based on measured mass and performance values for functional prototypes of my invention. These prototypes provided a measured 92% net optical efficiency and weighed less than 0.8 kilograms per square meter of lens aperture area.
In summary, my inflatable Fresnel lens invention overcomes prior art problems in two related, but different, space power fields: solar electric arrays and solar thermal concentrators. The innovativeness and usefulness of my new invention have been recognized by NASA by the selection of this new inflatable solar concentrator technology for development under the NASA Small Business Innovation Research (SBIR) program, specifically under Contract No. NAS397073.